Cyclotron wave tunable filter-constant gain parametric amplifier tube



Nov. 29, 1966 same MAQ 3,289,091

CYCLOTRON WAVE TUNABLE FILTER-CONSTANT GAIN PARAMETRIC AMPLIFIER TUBE Filed May 20, 1964 3 Sheets-Sheet 1 2 20 lo 22 23 24 B 56 25 RF. INPUT VARIABLE l GAIN REQ. 54 CONTROL 1 ONTROL w COLLECTOR) I GP 2/ W 22 ELECTRIC FIELD LINES AGE/VT Nov. 29, 1966 SHING MAO 3,28%)91 CYGLOTRON WAVE TUNABLE FILTER-CONSTANT GAIN PARAMETRIC AMPLIFIER TUBE Filed May 20, 1964 5 Sheets-Sheet 2 VOLTAGE I5 76 SOURCES l4 l7 MIXER TUBE VOLTAGES 0 0 1 /0 4P0 I i VOLTAGE l5 A SOURCES W6 2/222324 56 L 20 /6 /2 '990'0 v v v. 2 :WM' 1 f POWER DIVIDER D.C.PUMPING -P VOLTAGES J EE 4 0 74 E Pi F/G 5 nvvewron SH/NG MAO AGE/VT Nov. 29, 1966 SHING MAO CYCLOTRON WAVE TUNABLE FILTER-CONSTANT GAIN PARAMETRIC AMPLIFIER TUBE 5 Sheets-Sheet 5 Filed May 20, 1964 1 a FDQQ rozmDommm SATURATION mmioa .EDQEQ WAVE AMPLITUDE PHASE CONSTANHfi) VOLTAG E SOURCES 94 I /00 R. F. INPUT DC PUMPING VOLTAGES R.F. OUTPUT INVENTOH Sh'l/VG MAO m a? W AGE/VT United States Patent 3,289,091 CYCLOTRON WAVE TUNABLE FILTER-CON- STANT GAIN PARAMETRIC AMPLIFIER TUBE Shing Mao, Burlington, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed May 20, 1964, Ser. No. 368,835

- 2 Claims. (Cl. 3304.7)

This invention relates to electron beam devices and more particularly to a cyclotron wave device.

It is known that transverse modulation of anelectron beam which is confined in a longitudinal D.C. magnetic field forces the individual electrons to rotate in the transverse direction. If the beam has a-constant velocity in the longitudinal direction, then the transverse rotation of the electrons causes a helical trajectory. This combination of rotational translational motion of electrons can be considered as wave motion. The rate of rotational motion is determined by the D. C. magnetic field and is called the cyclotron frequency.

This cyclotron Wave phenomena has been utilized in the past to; great advantage in obtaining low noise parametric amplification of an R.F. wave. Known prior art devices ofthis type have in general required resonant input and output coupler circuits tuned to the cyclotron frequency for coupling R.F. signal energy into and out of the beam. Such couplers have a relatively narrow frequency bandwidth and severely limit the over-all bandwidth of these prior art devices.

Intermediate the input and output couplers is a pumping section wherein amplification of the R.F. signal on the beam occurs by the pumping action of either a high frequency transverse electric quadrupole field or a properly twisted D.C. quadrupole electrostatic field which fields serve to increase the transverse rotational radius of the beam resulting in amplification of R.F. modulations on the beam."

In the apparatus of the present invention wideband operation is made possible by simultaneously utilizing a single wideband dispersive structure, such as a quadrifilar helix, for coupling the R.F. signal into and out of the beam, and also as an active pumping structure. I

Furthermore, in accordance with the invention it has been found that by operating the above described quadrifilar helix, cyclotron wave amplifier in the mode, that is with the R.F. source input to each of the four helices of the quadrifilar helix being of equal amplitude and phase, an active tunable narrow band filter results. This novel active filter is quite stable since the 0- mode circuit waves have a relatively fast group velocity, close to the velocity of light, whereas the beamtravels much slower; accordingly, backward wave oscillations are inherently prohibited. The circuit wave group velocity is established by the pitch and radius of the helix and in contrast to conventional traveling wave devices is many times greater than the D.C. beam velocity. In other words, for a given electron beam velocity and helix diameter the helices have a much longer pitch than the conventional traveling wave tube helices of the prior art. Furthermore, gain may be controlled independent of signal frequency by variations in D.C. pumping voltage.

Additionally, it should be noted that although the intantaneous bandwidth of the filter is quite narrow, the center frequency can be electrically tuned over a wide range, for example, more than 1,000 mc., by varying the beam voltage and magnetic field.

Other objects, features and advantages of the invention will become apparent as the description progresses, reference being made to the accompanying drawings wherein like characters of reference designate like parts throughout the several views:

FIG. 1 is a partially schematic view of a cyclotron wave tube of the invention operated in the -0- mode;

3,289,091 Patented Nov. 29, 1966 FIG. 2 is a cross-sectional view through line 2-2 of FIG. .1;

FIG. 3 is a frequency versus phase constant (0: vs. 8) plot of the various circuit and beam waves of the device of FIG. 1;-

FIG. 4 is a partiallyschematic view of a cyclotron wave frequency mixer tube of the invention;

FIG. 5 is a partially schematic view of a cyclotron wave power divider tube of the invention;

FIGS. .6-8 are graphs useful for illustrating certain features of the invention; and

FIG. 9 is apartially schematic view of a severed helix embodiment of the invention.

Referring now to FIG. 1, there is shown a schematic illustration of an electronic beam device 10 comprising an electron gun 11 for forming and projecting an electron beam'27 along a path toward a collector 18. The electron gun is shown for illustrative purposes as comprising a cathode 14, a beam forming electrode 15, a first accelerating anode 17 and a second accelerating anode 16, which together with the collector 18 are maintained at predetermined potentials by voltage sources, such as batteries 41 through 44, respectively. The various electrodes are maintained within a vacuum by means of an envelope 12 which may be of glass or other suitable material. Surrounding the envelope is a cylindrical solenoid 25 powered by current from current sources 46 and 48. The cylindrical solenoid produces a longitudinal magnet field, B, for focusing" the beam and establishing a cyclotron mode of wave propagation within the beam.

A quadrifilar helix 20 comprising four helices 21-24 of substantiallythe same diameter and electrically insulated and equally spaced one from the other is disposed within.

the tube envelope 12 adjacent and coaxial to the electron beam 27. R.F. input energy is coupled through capacitors 28 and 30 to each of the four helices 21-24 comprising the quadrifilar helix. The R.F. energy input at each of the helices is of substantially the same amplitude and phase. This condition of equal phase and amplitude R.F. signals corresponds to the 0- mode of excitation as contrasted to, for example, the 1 mode of excitation in which the input R.F. energy has a phase shift in succeeding adjacent helices. The D.C. pumping energy is coupled from batteries 53 and 54 through coils 51 and 52 respectively which form an R.F. block to each of the four helices of the quadrifilar helix. Helices 22 and 24 are thus provided with a positive potential D.C. pumping signal from battery 54, whereas helices 23 and 21 are provided with a negative potential D.C. pumping signal from battery.53. This D.C. pumping scheme places two opposite 90* sectors A and C of the beam 27 continually in a transverse accelerating field and the two remaining adjacent 90. sectors B and D in a transverse decelerating field as shown in FIG. 2 which is a cross section view plained by considering FIG. 1 in connection with the dispersion curves shown in FIG. 3.

FIG. 3 is a plot of the phase constant B of the circuit and beam waves versus the angular frequency w of the waves. The dotted line of curve a represents the (WB) characteristics of the 0 mode circuit wave. The solid lines of curves b and c represent the (w-fi) characteristics of the fast and slow cyclotron beam waves respectively.

The helix 20 is designed to have a synchronous velocity much higher than the beam velocity. Coupling between the 0 mode circuit wave and the fast cyclotron wave occurs only in a very narrow frequency range; for example, at the intersection of curves a and b of FIG. 3. Thus, R.F. energy, containing a wide band of frequency signals,

v the helix.

introduced at the input of quadrifilar helix 20 is filtered upon passage through device 10 since only the RF. energy at the frequency denoted by the intersection of curves a and b will interact with the fast cyclotron wave. The R.F. signal coupled into the fast cyclotron wave is amplified by the DC. pumping field represented by the vector (NP, B on the helices which pumps energy from the slow cyclotron wave c into the fast cyclotron wave b as long as the parametric relations wp=w w and fi f3 p are satisfied. In other words, the pumping frequency w must equal the diiference between the RF. signal frequency a an idler frequency to; (slow wave frequency in this case)..- Furthermore, the phase constant ti of the pumping field must equal the difference between the phase constant of the RF. signal frequencyfi and the phase constant of the idler frequency It should be noted that the phase con--- stant B of a wave is equal to the ratio of wave frequency over wave phase velocity. I

The above parametric relations are shown asa vectorv sum in FIG. 3. Thus, the condition is satisfied when: vector (m 13 is the vector sum of vectors (w B and I vector (u 5 as shown in FIG.- 3.

R.F. signals on the helices which arev at different frequencies other than the; cross-over frequency are attenuated by loss y material 56 in contact with the helix 20.; It should be noted that the R.F. energy at the selected frequency will not be attenuated by the lossy material 56 since R.F. energy at that frequency has been coupled into the beam and will pass through the attenuating section 56 on the fast cyclotron beam Wave rather than on The selected signal which passes through the attenuating section on the beam continues to be pumped by the Y pumping field on the helix and interacts with the, helix circuit so that the amplified signal couples back to the RF. structure. The amplified R.F. signal is extracted fromthe four helices comprising the quadrifilar helix by means of coupling capacitors 34- and 32 coupled to RF. output terminal 58. It may thus be seen that the helix 2!) provides both .input and output coupling as well as serving as a pumping structure. The beam is ultimately collected by beam collector electrode 181 .The apparatus of FIG. 1 as thus described performs the combined function of a voltage tunable narrow band filter followed by a wide band amplifier on one structure. In other words, amplification is obtained from the same device which provides the filtering action. In operation, separate gain control can be achieved by varying the DC. pumping voltages by means of gain control knob 26. Frequency selection is obtained by varying the strength of the magnetic field by means of frequency control knob 24 coupled to variable current source 48 which in turn is in parallel with fixed current source 46, both of which supply operating current to solenoid 25. Varying the magnetic fieldmoves curve b of FIG. 3 up or. down depending on the direction in which'themagnetio I field is changed and hence changes the synchronism frequency. i v

-In order to maintain constant gain .with variable frequency control it is necessary to simultaneously vary the magnetic field while varying the beam voltage. A change in beam voltage changes the slope of curve b of FIG. 3. This slope must be changed in unison with frequency change so as to maintain the parametric relationships previously mentioned. Thus,-.there is provided in accordance with the invention a clutch mechanism 60 intermediate frequency control knob 24 and variable voltage source 43. Thus, frequency control knob 24 may be coupled both to current source 48 and voltage source 43 simultaneously by positioning the frequency control knob downwards inadetent' action, thereby enabling constant gain frequency selection.

In the apparatus of FIG. 4 a further embodiment of the cyclotron wave quadrifilar helix tube of the invention is shown wher l b? tube is utilized as a mixer.. .In FIG.

4 there is shown acyclotron-wave tube lfl-su'bstantially" F to helix '21. The helix may be excited, by source 74 in either of four possible circuit modes. These modes are the -O, "-2; 1 and +1 modes. The zero mode as aforerelated corresponds to an excitation of the four helices with a signal of the same amplitude and phase.

1 In-the 2 mode of operation the fourhelices are excited by equal amplitude signals in amanner such that it the signals on adjacentfhelices are 180 degrees out of phase. The l mode requires equal; amplitude signals, coupled such that succeeding helices are coupled to signals progressively 90 degrees apart-in phase,- The +l 1 mode is identical to the 1 mode except that the phase progression is in the opposite direction.

--FIG. 7 shows the w-fl or propagation characteristics r of the four circuit modes. FIG. Sshows the w char- ';acteristics of transverse waves carried by an electron beam confined in a. longitudinal D.C. magnetic field. From'an analysisof FIGS. 7 and 8 it, can be seen that by suitably designing the helix dimensions any of" the circuit modedispersion curves can be made to coincide-with;- any one ;0f the beam modes throughout an extended-frequency range resulting in couplingtherebetween.

For purposesl of simplicity'and consistencyonly the -.0 mode type of excitation and coupling -is discussed ,in' connection-with FIGS. 4, 5, 6 and 9. I r

' Referring again to FIG..4, the RI E signalxF which may for example be a radarlocal oscillator signal, propagates along helix 23. The F signal which may, for. example, comprise a high. frequency radar echosignal i propagates along helix 21-. Each signal interacts with 1 or couples into .fast cyclotron waves on the beam. Thepumping voltages from source 74 are maintainedisuch thatthe tube is'operated at the saturation level indicated in FIG. 6. T he RF. signals F and F which have been coupledinto the beam interact in' a nonlinear fashion to produce'sum and difference frequency signals on the fast cyclotron beam waves. -Lossy material 56 intermediately disposed on a "plurality of turns of helix 20-absorbs the from'the slowwave to the 'fast wave. -The amplified signals couple back onto the helix and are extracted and coupled through coupling capacitors 70 and 32 to suitable load means not shown which may comprise, for example,

a radar receiver.

It should be noted that the apparatus of FIG. 4 diiiers from conventional mixer tubes in that power saturation is in the transverse direction, whereas in conventional mixer tubes saturation is in the longitudinal direction. In addition the apparatus of FIG. 4 can accept any level input signal since the/DC. pumping 'signalsact as biases which can'be' adjusted according to the' level ofthe input signalto achievepropermixing'.

I In the apparatus of FIG. 5 an input signal P is divided into four separate signals, each having'one-fourththe power of the output signal. The input signal is filtered as i in FIG. 1 and amplified as in FIG. 4... Such a' system.

eliminates the extra power. divider required in phased array and telemetry equipment. As can be seen in FIG. 5, the input signal P is coupled D .C. wise to only one of the four helices comprising the helix 20; however,

interaction between the circuit and beam waves occurs R.F. wise withall four helices as described in connection with FIGS: 1 and 4. The amplified. and/ or filtered signal is extracted at the end of the helix 20 in the form of four signals one-fourth power, each of which is onequarter of the power of the output signal.

As an alternative to providing lossy material 56 on the helices 21-24 of the embodiments of FIGS. 1, 4, 2 and 5, a bifilar helix may be utilized to initially couple the circuit waves to the beam waves. This embodiment is shown in FIG. 9 wherein the R.F. signal is coupled into the fast cyclotron beam wave by the bifilar helix 90 comprised of helices 91 and 92 coupled through capacitor 94 to R.F. input terminal 100. By proper selection of helix, pitch coupling between beam and bifilar helix can be either broad band for broad band amplification or narrow band for filtering. Noise energy carried by the fast cyclotron wave couples out of the beam and terminates at loads 101 and 102 of the bifilar helix 90.

The R.F. signal is carried by the beam into the quadrifilar helix region. The quadrifilar helix 20 is provided with an active electrostatic pumping field from pump circuit 99 which transfers power from the slow cyclotron Wave to the fast cyclotron wave. The fundamental circuit wave, which may be a mode, +1 mode, 1 anode or 2 mode, couples with the fast cyclotron wave and couples the amplified R.F. signal out of the beam 27 to helix 20 and to R.F. output terminal 103 by way of coupling capacitors 95-98.

It is to be understood that the foregoing embodiments are only illustrative of the inventive concepts involved. Various other arrangements may be made by those skilled in the art without departing from the spirit :and scope of the invention as set forth in the following claims.

What is claimed is:

1. In combination:

means for forming and projecting a beam of electrons;

means for producing a longitudinal magnetic field along the beam;

a quadrifilar helix comprising four helices of equal diameter and pitch equally spaced from one another and coaxial to said beam for coupling R.F. circuit energy excited in the -0 mode into and out of the beam and for providing a twisted quadrupole D.C. electrostatic field about the beam;

and means for simultaneously varying the beam voltage and magnetic field strength to thereby yield a velocity synchronous interaction relationship between the R.F. circuit wave and the fast cyclotron beam wave in a predetermined narrow frequency range.

2. In combination:

means for forming and projecting a beam of electrons;

means for producing a longitudinal magnetic field along the beam;

a quadrifilar helix comprising four helices of equal diameter and pitch equally spaced from one another and coaxial to said beam for coupling circuit energy into and out of the beam and for providing a twisted quadrupole electrostatic field about the beam;

means for supplying a positive DC voltage to an alternate pair of said helices;

means for supplying a negative DC. voltage to the remaining pair of helices;

means for coupling R.F. input energy connected to the end of each of said helices nearest the means for forming electrons;

means for coupling R.F. output energy connected to the end of each of said helices furthest removed from the means for forming electrons;

means for simultaneously varying the beam voltage and and magnetic field strength to thereby yield a velocity synchronous interaction relationship between the R.F. circuit wave and the fast cyclotron beam wave at a predetermined cross-over frequency;

and means coupled to said helices intermediate opposite ends of said helices for attenuation of R.F. signal energy at frequencies other than said predetermined frequency.

References Cited by the Examiner UNITED STATES PATENTS 3,059,138 10/1962 Wade 3304.7 3,212,017 10/1965 Sackinger 3304.7 3,221,264 11/1965 Alder 330-'4.7 3,231,825 1/1966 Forster et a1. 330-4] ROY LAKE, Primary Examiner.

DARWIN R. HOSTETTER, Assistant Examiner. 

1. IN COMBINATION: MEANS FOR FORMING AND PROJECTING A BEAM OF ELECTRONS; MEANS FOR PRODUCING A LONGITUDINAL MAGNETIC FIELD ALONG THE BEAM; A QUADRIFILAR HELIX COMPRISING FOUR HELICES OF EQUAL DIAMETER AND PITCH EQUALLY SPACED FORM ONE ANOTHER AND COAXIAL TO SAID BEAM FOR COUPLING R.F. CIRCUIT ENERGY EXCITED IN THE -O- MODE INTO AND OUT OF THE BEAM AND FOR PROVIDING A TWISTED QUADRUPOLE D.C. ELECTROSTATIC FIELD ABOUT THE BEAM; AND MEANS FOR SIMULTANEOUSLY VARYING THE BEAM VOLTAGE AND MAGNETIC FIELD STRENGTH TO THEREBY YIELD A VELOCITY SYNCHRONOUS INTERACTION RELATIONSHIP BETWEEN THE R.F. CIRCUIT WAVE AND THE FAST CYCLOTRON BEAM WAVE IN A PREDETERMINED NARROW FREQUENCY RANGE. 